TECH TIP: 220...240 -- Whatever It Takes!


When you're tasked with selecting a frame rate for your seamless switcher's output, many people adopt the same attitude as Jack Butler (Michael Keaton's character in the 1983 film Mr. Mom). To paraphrase Mr. Butler's memorable quip, "59.94 Hz or 60 Hz...whatever it takes." In most professional seamless switchers, there's an excellent reason why a choice is provided between these two (seemingly close) frame rates -- and the reason is not quite as random as "whatever it takes."

Frame rate is the frequency at which imaging devices produce unique consecutive pictures, which we refer to as frames. This definition applies to images produced by video cameras, film cameras, computer graphics systems, and motion capture systems. In the film world, frame rate is expressed in frames per second (fps), while in the video and computer graphics realm, frame rate is expressed in hertz (Hz).

The 59.94 Hz frame rate comes from an NTSC (National Television Standards Committee) standard, and it's the refresh rate used for all standard definition video in the United States and numerous countries worldwide. Originally, back in "monochrome" days, the frame rate matched the power line frequency (60.00 Hz), but when color television came along, the frame rate was changed to 59.94 Hz. The FCC made this change in order to accommodate chrominance data within the interlaced video signal, to avoid interference with the audio signal, and, more important, so as not to anger consumers who had already purchased television sets. (If you'd like to get a headache and learn more about the FCC's decision back then, simply Google "Why is NTSC 59.94" and read all about it.)

While CRT (Cathode Ray Tube) television sets are now in their sunset years, we get to keep the legacy NTSC standard that was specifically designed for them.

When computer graphics cards and monitors were designed in the '80s, the available technology allowed for faster refresh rates and the use of non-interlaced (progressive) video. As technology advanced, progressive video formats migrated into projectors and large format display monitors.
The dilemma, when using a seamless switcher in a professional presentation system, is that the interlaced camera and other NTSC signals are routed to the switcher at 59.94 Hz, but the switcher's output is routed to a progressive display. If the target display is set to 60 Hz, this seemingly small timing difference results in a "jutter" artifact -- clearly evident as a pulsating or stutter effect on screen, or as a line that moves vertically across the image.

The optimum way to deal with these artifacts is to set the switcher's output timing to match the refresh rate of the NTSC input sources (at 59.94 Hz). The key is that this frame rate is independent of the target display's resolution. For example, you can set the output resolution to 1024 x 768 at 59.94 Hz or 1280 x 1024 at 59.94 Hz (rather than 60.00 Hz), and the display will automatically adapt to the incoming frequency. By using the NTSC refresh rate, the jutter artifacts are avoided.